What does underweight mean?
Being underweight means you are below the healthy weight range — the weight that research has shown you need to be if your body is to work properly. Being outside this range, whether overweight or underweight, can seriously affect your health.
What are the symptoms of being underweight?
Some people who are underweight find they get sick all the time, or they feel very tired. That may be because they are not getting all the nutrients they need from their diet. They may also find their hair gets thinner or falls out, their skin gets very dry and their teeth are affected.
Children who are underweight may not grow as expected for their age.
Being underweight can lead to health problems including:
- infections, which are harder to fight off
- irregular periods in women, or menstruation may stop altogether
- pregnancy problems, such as difficulty falling pregnant or a higher likelihood of preterm labour
- heart problems
Being underweight can be especially dangerous for older people since it increases their risk of breaking a bone and becoming ill.
What causes being underweight?
People become underweight for many different reasons. Some may be born naturally small and their low BMI is due to their genes. Some may have a very high metabolism and find it hard to put on weight, even if they eat foods that have a lot of calories.
Some people may not follow a healthy, balanced diet because they forget to eat or they cannot afford nutritious foods. Others may not be eating properly because they are sick, or their medicines make them feel nauseous. Some people who do a lot of physical activity burn up more calories than they can eat, leading to their being underweight.
Some physical conditions can cause weight loss, leading to someone being underweight. These include:
- stomach problems, such as nausea, vomiting or diarrhoea
- an infection
- cancer, HIV or lung disease
- thyroid problems
- Crohn's disease
Mental health problems can also cause someone to be underweight. These include:
People who have been ill, in hospital or who are elderly are more at risk of being underweight.
How do I know if I am underweight?
The best measure of whether you are a healthy weight is the body mass index (BMI). This is calculated using your weight and your height. In adults, a healthy weight range is a BMI of 18.5 to 24.9. If your BMI is under 18.5 then you would be considered underweight.
Note that BMI calculations do not apply in the same way to some people. For example, athletes, people of some ethnic backgrounds and pregnant women should not rely on their BMI to calculate whether they are underweight. If you think you might be in one of these groups, ask your doctor for advice.
The weight of children is usually measured by growth charts. These calculate how a child is growing against what would be expected for other girls and boys of their age.
What are the risks of being underweight?
If you are underweight, you may be at greater risk of certain health conditions, including malnutrition, osteoporosis, decreased muscle strength, hypothermia and lowered immunity. You are more likely to die at a younger age.
Underweight women have less chance of becoming pregnant than women who are a healthy weight.
How can I gain weight safely?
If you are underweight, it is important to eat a variety of foods that give you the nutrition you need. You should make sure you eat enough energy to gain weight, protein to repair your body and build your muscles, and vitamins and minerals to make you healthy.
The aim is to gain weight gradually by eating healthy foods. Even if you are underweight, try to avoid foods with a lot of added sugar, fat and salt, like cakes, takeaway foods and sugary drinks.
You can put on weight by eating small meals frequently throughout the day. Try to snack on healthy, high energy foods like cheese, nuts, milk-based smoothies and dried fruit.
Further tips for gaining weight safely include:
- using full cream milk
- using healthy fats like olive oil or avocado
- sprinkling some grated cheese on cooked food
- adding skim milk powder to soups, stews and drinks
- adding protein powder to milkshakes
- replacing tea or coffee with milky drinks
- doing some exercise to increase your appetite
- having meals delivered if you have difficulty preparing them yourself
If you are having trouble gaining weight, it is best to speak to a doctor to rule out an underlying cause for your underweight.
If you have an eating disorder, it is very important to seek treatment from a health professional. Talk to your doctor or contact The Butterfly Foundation for more information. You can call them on 1800 33 4673, 8am – midnight AEST, 7 days a week. Alternatively, you can chat online or email them.
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Last reviewed: May 2021